400 THE WORLD OF ANIMAL LIFE 



As we should naturally suppose, the land group of Molluscs are 

 almost without exception air-breathers, and are therefore formed 

 into the order Pulmonata. 



The marine group are almost entirely water-breathers, and so 

 they form another order, the Pectinibranchiata, i.e. the "comb- 

 gilled ", because the plates of their gills are arranged like the teeth 

 of a comb. 



But many of the freshwater Molluscs are lung-breathers, like 

 their friends on land ; they have therefore to come to the surface of 

 the water occasionally to renew the supply of oxygen. Some of 

 the aqueous Mollusca have a lung and gills as well. Others have 

 gills which grow on the outside of their bodies like small trees or 

 shrubs ; but we need not say more here on this head. Let us look 

 at our friend the " snail ". 



SNAILS (Family Helicid^) 



Most of us think the Snail a very mischievous creature; and in 

 our gardens no doubt it is. It eats our vegetables, flowers, and 

 fruits, and does some harm; but in the woods and fields it is not 

 mischievous, and there is no reason why we should destroy it. 



It is a much more interesting creature than we might suppose; 

 and there is a good deal for us to learn about it. 



Let us suppose that we have a snail crawling upon the table in 

 front of us. We shall see that the part of the animal outside the 

 shell is long and shiny, and that it has a kind of head with four 

 horns, two of which are much longer than the others. 



The part of the body upon which it crawls is called the foot, 

 because it makes progression by it. As it crawls along you will 

 notice that it first stretches out the front part, and then draws up 

 the rest after it ; so that it really moves somewhat like a worm. 



The two longer horns, or feelers, upon the head have each 

 a little black speck at the tip. This speck is an eye, and the snail 

 can push it out and withdraw it when it wishes, besides being able 

 to draw the feelers themselves back into the head. 



The snail cannot make its way upon a dry surface. As it 



